Fix rust-analyzer selecting the wrong proc-macro cfg

rust/private/rust_analyzer.bzl

@@ -20,7 +20,7 @@ given targets. This file can be consumed by rust-analyzer as an alternative
 to Cargo.toml files.
 """
 
-load("//rust/platform:triple_mappings.bzl", "system_to_dylib_ext", "triple_to_system")
+load("//rust/platform:triple_mappings.bzl", "system_to_dylib_ext")
 load("//rust/private:common.bzl", "rust_common")
 load("//rust/private:providers.bzl", "RustAnalyzerGroupInfo", "RustAnalyzerInfo")
 load("//rust/private:rustc.bzl", "BuildInfo")
@@ -195,14 +195,13 @@ def find_proc_macro_dylib(toolchain, target):
     if crate_info.type != "proc-macro":
         return None
 
-    dylib_ext = system_to_dylib_ext(triple_to_system(toolchain.target_triple))
+    # Use the exec ext: rust-analyzer (host) is what loads the dylib.
+    dylib_ext = system_to_dylib_ext(toolchain.exec_triple.system)
     for action in target.actions:
         for output in action.outputs.to_list():
             if output.extension == dylib_ext[1:]:
                 return output
 
-    # Failed to find the dylib path inside a proc-macro crate.
-    # TODO: Should this be an error?
     return None
 
 rust_analyzer_aspect = aspect(

test/rust_analyzer/proc_macro_dylib_test/BUILD.bazel

@@ -0,0 +1,13 @@
+load("@rules_rust//rust:defs.bzl", "rust_proc_macro")
+load(":proc_macro_dylib_test.bzl", "exec_triple_selects_dylib_test")
+
+rust_proc_macro(
+    name = "test_proc_macro",
+    srcs = ["test_proc_macro.rs"],
+    edition = "2021",
+)
+
+exec_triple_selects_dylib_test(
+    name = "exec_triple_selects_dylib",
+    target_under_test = ":test_proc_macro",
+)

test/rust_analyzer/proc_macro_dylib_test/proc_macro_dylib_test.bzl

@@ -0,0 +1,51 @@
+"""Regression test: find_proc_macro_dylib must key off exec_triple.
+
+Proc-macros are loaded by the host rust-analyzer process, so the dylib
+extension used to locate the compiled artifact must match the exec platform,
+not the target — otherwise a cross-compile spec points at an unloadable
+artifact (e.g. `.wasm` or `.dll` on a Linux host).
+"""
+
+load("@bazel_skylib//lib:unittest.bzl", "analysistest", "asserts")
+
+# buildifier: disable=bzl-visibility
+load("@rules_rust//rust/private:rust_analyzer.bzl", "find_proc_macro_dylib")
+
+_ProcMacroDylibProbeInfo = provider(
+    doc = "Captures find_proc_macro_dylib results across mock exec triples.",
+    fields = ["by_system"],
+)
+
+def _probe_aspect_impl(target, ctx):
+    if ctx.rule.kind != "rust_proc_macro":
+        return []
+
+    # Pairwise-distinct dylib extensions: .so / .dylib / .dll / .wasm.
+    by_system = {
+        system: find_proc_macro_dylib(
+            struct(exec_triple = struct(system = system)),
+            target,
+        )
+        for system in ["linux", "macos", "windows", "wasi"]
+    }
+    return [_ProcMacroDylibProbeInfo(by_system = by_system)]
+
+_probe_aspect = aspect(implementation = _probe_aspect_impl)
+
+def _exec_triple_selects_dylib_impl(ctx):
+    env = analysistest.begin(ctx)
+    probe = analysistest.target_under_test(env)[_ProcMacroDylibProbeInfo]
+
+    hits = [s for s, d in probe.by_system.items() if d != None]
+    asserts.equals(
+        env,
+        1,
+        len(hits),
+        "Expected exactly one exec_triple.system to find the host proc-macro dylib; got {}".format(probe.by_system),
+    )
+    return analysistest.end(env)
+
+exec_triple_selects_dylib_test = analysistest.make(
+    _exec_triple_selects_dylib_impl,
+    extra_target_under_test_aspects = [_probe_aspect],
+)

test/rust_analyzer/proc_macro_dylib_test/test_proc_macro.rs

@@ -0,0 +1,8 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_derive(Marker)]
+pub fn marker(_input: TokenStream) -> TokenStream {
+    TokenStream::new()
+}